Pedicle screw system for osteosynthesis

ABSTRACT

The invention is a pedicle screw assembly ( 10 ) for use with a rod ( 11 ) for the immobilization of bone segments. The assembly is composed of a screw ( 12 ), a poly-axial housing ( 20 ), a washer ( 34 ), a set screw ( 48 ), and a cup shaped washer ( 40 ). When the screw is placed inside the poly-axial housing ( 21 ), the head of the screw ( 12 ) comes into contact with a middle section of the poly-axial housing ( 18 ), and is secured into the bone so that the poly-axial housing is pivotable. The housing includes a pair of upstanding posts ( 38 ) with interior threads. A washer ( 24 ) is inserted between the head of the screw ( 12 ) and the rod. A cap ( 40 ), having a bottom with a pair of openings ( 43 ) and a lateral cross connector ( 44 ), is placed over the posts ( 38 ) so that the cross connector ( 44 ) engages the rod. The cross connector and washer have semi-cylindrical rod engaging surfaces. A set screw ( 42 ) is threaded into the housing posts to secure the rod.

RELATED APPLICATION

This application is a continuation-in-part of my application Ser. No.08/771,133 filed Dec. 20, 1996 entitled Pedicle Screw System forOsteosynthesis, now issued U.S. Pat. No. 5,782,833 and further claimspriority of PCT/US97/23851 filed Dec. 19, 1997.

TECHNICAL FIELD

The present invention relates to the medical field commonly referred toas Osteosynthesis, i.e., the fusion between segments of the spine andmore particularly to a pedicle screw and rod system for immobilizing thesegments during the fusion process.

BACKGROUND ART

Osteosynthesis is achieved by immobilizing the bone. When trying toachieve osteosynthesis and specifically fusion between differentsegments of the spine, one has to provide some type of immobilization.There are various prior art systems which try to achieve this purpose.The different systems involve placement of screws into the bone. Thescrews are then connected to each other by use of various sizes of rodsor a plate. The bone segments that are being connected, especially inthe spine, may be carrying different angles and differentmedical-lateral positions. Placement of a rod with a rigid screw orplacement of a plate between two rigid screws is difficult because ofthe medial lateral displacement or angulation at different segments. Onehas to bend the rod or plate and at times achieve a complex bend inorder to connect two different segments of the bone and especially twodifferent areas in the spine. When dealing with the spine, the screwsare ordinarily placed into the Pedicle, and due to the differentpositions of the pedicle and different angulations of the screw as itenters the pedicle, one encounters difficulty in positioning andconnecting these screws at various points.

Even though one can create a complex bend at the rod or the plate inorder to connect two or more screws, there are places in the pediclewhere one runs the chance of stress risers at different points andbreakage of the system as the bends can never be perfect.

A screw system, which is capable of accommodating the rod in a perfectlocation without creating any appreciable areas of stress riser, willalleviate some of the above problems. Such a screw system would allowthe rod to be bent to achieve fixation between two different pointswhile adjusting to any imperfections in the bend.

There is at least one polyaxial screw system that has been used in thepast which will achieve some of these goals; however, there are someinherent problems with this particular system. This polyaxial screw hasmany components which makes placement of such a screw cumbersome, whichin turn, lengthens the operative time for this particular procedure. Thesystem has a locking screw on the inside as well as a locking nut on theoutside of the housing, which causes the operation to take much longerto perform. The fixation point which will lock the polyaxial screw andkeep it from angling once the system is tightened is also not ideal.

Several patents teach the use of a pedicle screw system which appear toprovide several degrees of freedom (i.e., rotation and limited angulardeflection about a fixed point) for the immobilization of bone segments.See for example U.S. Pat. No. 5,360,431 to Puno et al, U.S. Pat. No.5,443,467 to Biedermann et al and U.S. Pat. No. 5,176,678 to Tsou. Eachof these patented structures has certain drawbacks including the use ofa conventional nut to secure the rod into place for support of the bonesegments. The nuts have flat surrounding edges which are engaged by awrench to tighten the nut. Due to the surrounding tissue, and theconfined area, difficulty can arise in placing the nut in the correctposition thus requiring even more time to perform the operation. Duringthe operation the patient is under anesthesia and this extra timeincreases the risk to the patient. Also, when secured the nut protrudesinto the surrounding soft tissue after the operation is completed. Thisprotrusion can lead to irritation of the surrounding soft tissue andpossibly inflammation.

Another problem arising with the use of the nut is the tighteningprocess. The nut is secured through the use of a wrench. The wrenchrequires space around the nut to be operable which necessarily increasesthe scope of the surgical procedure. Furthermore, the wrench should notcome into contact with the surrounding soft tissue to avoid thepossibility of peripheral tissue damage. These limitations tend tofurther increase the risk to the patient during the operation.

Yet another problem with the prior art systems involves the manner inwhich the screw system is affixed to the rod. For example, systems likethat disclosed in the Biedermann et al patent utilize washers or nuts oneach side of the rod to secure the rod to the housing. The nuts orwashers have planar surfaces which make contact with the rod only alongsegments of a line. Such a minimal contact with the rod will not providesufficient purchase on the surface of the rod to prevent post operativemovement between the rod and housing. Unless the screw housing is firmlygrasps the rod the rod will have a tendency to twist or rotate inside ofone or more of the housings. If the rod is allowed to rotate relative toone housing (and associated screw) while an adjacent housing and screwremain locked to the rod the screw secured to the locked housing maytravel and break out of the vertebrae in which it was embedded. Thiswill result in a damaged vertebrae and perhaps a severed nerve exitingthe spinal column at that point. If the rod is allowed to rotaterelative to all of the several housings to which it was originallysecured it may migrate into muscle, soft tissue or even into the spinalcolumn itself.

There is a need for a more reliable pedicle screw and rod system whichmay readily and rapidly be secured in place, with less bulky equipmentand which is less intrusive to the surrounding soft tissue.

SUMMARY OF THE INVENTION

The present invention addresses the stabilization of bone segmentsthrough the use of a polyaxial pedicle screw assembly and rod. The rodis arranged to be secured between two or more embedded screw assembliesto immobilize segments of the spine. The assembly includes a screw whichhas a head and a threaded cylindrical shaft and is threaded into thebone. The head of the screw has a top and bottom, both of which arespherically convex in shape with the head being larger than the diameterof the cylindrical shaft. The top of the screw head has a wrenchengaging surface, such as an allen wrench socket.

The screw fits within a polyaxial housing having a stepped bore adaptedto receive the rod. The polyaxial housing is divided into threesections. The top section of the housing receives the entire screwincluding the head and is formed by a pair of spaced upstanding postswhich define a U-shaped slot therebetween for receiving the rod. Theinner walls of the posts are threaded for receiving a set screw whichsecures the rod in place. The middle section of the housing has an innerspherically concave surface for cradling the bottom of the head of thescrew. The bore through the bottom section has a diameter which allowsonly the threaded cylindrical shaft to pass through. The screw, afterinsertion into the polyaxial housing, is threadably secured into thebone.

A washer with a generally spherically concave bottom surface to engagethe head of the screw is then placed within the housing. Preferably thebottom surface of the washer is provided with a roughened surface, suchas asperities in the form of sharp edges to provide a locking actionbetween the washer and the screw head in the assembled condition. Thetop of the washer is provided with a concave surface, preferablysemicylindrical or saddle shaped, to conform to the shape of the rod.

The lower surface of the washer provides for a positive gripping surfacearea with the screw head thereby adding to the stability of the rod andscrew, once in place.

The screw assembly further includes a cup-shaped cap having two opposingopenings to receive the posts and a cross-connector extending across thebottom of the cap. The cross-connector has a flat top and a bottom witha concave semicylindrical or longitudinal saddle shape to conform to theshape of the rod. The cup-shaped cap is adapted to be placed over thepolyaxial housing with the bottom surface of the cross-connector makingcontact with the rod.

A set screw of conventional configuration is arranged to be threadedinto the top section of the polyaxial housing by means of a wrenchinserted into a wrench engaging surface, such as a allen wrench socket,in the top of the set screw, to tighten the assembly into place. The rodengaging surface of the cross connector may be provided with asperitiesformed, for example, by a series of sharp ridges (or edges) runningparallel to the longitudinal axis of the rod or formed with a smallerradius of curvature at the bottom than at the top so that the crossconnector will firmly grasp the rod and prevent it from twisting withinthe housing when the set screw is tightened.

With the set screw in place, but not tightened, the assembly has threedegrees of freedom, i.e., rotatable and angularly positionable about thehead of the screw. The tightening of the set screw secures the assemblyinto a single position. The set screw allows the assembly to betightened while overcoming the disadvantage of potential soft tissuedamage due to the use of a nut. The screw assembly of the presentinvention when secured in place does not protrude into the surroundingsoft tissue and thus reduces the risk of irritation and soft tissuedamage.

The present invention provides a highly flexible and stable bone segmentimmobilization system with a minimum number of components which resultsin a reduction in the time that a patient must remain under anesthesia.

The construction and operational features of the present invention maybest be understood by reference to the following description taken inconjunction with the appended drawings in which like components in theseveral figures are identified by the same reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of several segments of a spiral columnwith a pedicle screw assembly and rod, in accordance with the presentinvention, secured thereto;

FIG. 2, is a cross-sectional view of the assembled pedicle screwassembly taken along lines 1—1 of FIG. 1, showing only a portion of thescrew shaft;

FIG. 3 is a cross-sectional view of the assembly taken along lines 3—3of FIG. 2, showing the top of the washer, the housing posts and thelower end of the cap, but not the rod;

FIG. 4 is an exploded view of the screw assembly and rod;

FIG. 5 is a cross-sectional view of the washer taken along lines 5—5 ofFIG. 4;

FIG. 6 is a cross-sectional view of an alternative embodiment of theassembly showing only a portion of the screw shaft;

FIG. 7 is an exploded view of the assembly of FIG. 6;

FIG. 8 is a cross-sectional view of the assembly taken along lines 8—8of FIG. 6;

FIG. 9 is a partial cross sectional view showing an alternative designfor the rod engaging surface of the cap cross connector;

FIG. 10 is a bottom plan view of the alternative cap arrangement shownin FIG. 9; and

FIG. 11 is a bottom plan view of the washer utilized in the embodimentof FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and particularly to FIG. 1, a pediclescrew assembly 10, in accordance with the present invention, is intendedto be secured in bone segments of a patient's spine and in conjunctionwith a rigid (though bendable) rod 11, to immobilize and allow thesegments to fuse together.

As is illustrated in FIGS. 2 and 4, the screw assembly 10 includes apedicle screw 12 which has a cylindrical shaft 14 threaded at 15, and ahead 16 formed integrally with the shaft. The head of the screw isspherical, except for the junction of the shaft with the head. Thespherical surface of the head of the screw is divided into upper andlower surfaces with each surface (i.e., lower 17 and upper 18) beinggenerally semispherical. The top of the head of the screw includes anindentation in the form of a wrench engaging surface 19. The wrenchengaging surface 19 is illustrated in the form of a hexagonal socket forreceiving a suitable wrench such as an allen wrench. The wrench engagingsurface could also be in the form of a slot for receiving a screwdriveror any other suitable recessed shape. The head of the screw has adiameter which is larger than the diameter of the threaded shaft 14 asis illustrated in FIG. 2.

A polyaxial housing 20 for receiving the screw, is composed of a top 22,a middle 24 and bottom section 26, as shown in FIG. 2. A stepped axialbore 28 extends through the housing with the bore in the top section 22having a diameter larger than the diameter of the threaded shaft of thescrew. The top section defines a pair of upstanding posts 30 with a rodreceiving slot 31 therebetween. The inside walls 32 of the posts arethreaded to receive a set screw, to be described. The middle section 24of the housing defines an inner generally spherically concave surface 27adapted to engage the convex semi-spherical bottom surface 17 of thehead of the screw.

The bore in the bottom section 26 of the housing has a diameter which isgreater than the diameter of the threaded cylindrical shaft 14, butsmaller than the diameter of the head 16 of the screw. During assemblythe screw is placed within the polyaxial housing 20 such that the bottom17 of the head of the screw 12 comes into contact with the sphericallyconcave portion of the middle section 27, as is illustrated in FIG. 2.The head of the screw is thus captured or retained within the lowerportion of the housing 20. The top section 22 of the housing has acylindrical outer surface 33, interrupted by the slot 31 and the middleand bottom sections have a generally spherical exterior surface 39 whichextends from the upper surface 33 to the bore 28.

A washer 34 is adapted to be placed between the rod and the uppersurface of the head of the screw as is illustrated in FIGS. 2 and 4. Thetop surface of the washer 34 includes a generally concavesemicylindrical rod engaging surface 36 which receives the rod 11. Thebottom of the washer has a generally spherically concave screw headengaging surface 38 for engaging the top surface 18 of the head of thescrew 12. See FIG. 5.

A cup-shaped cap 40, having a cylindrical outer and inner surface 41 and42, respectively, is adapted to be placed over the polyaxial housing 20and engage the top of the rod 11, as is illustrated in FIGS. 2 and 4.The cap 40 has a bottom with a pair of spaced arcuate openings 43 forreceiving the posts 30 and a cross-connector 44 spanning the lateralwidth of the lower inside surface of the cap between the openings. Thecross-connector 44 has a top 46 that is flat and a bottom 46 a thatincludes a longitudinal concave rod-engaging surface 47 (i.e.,semicylindrical, saddle or U-shaped surface). The cap is adapted to beplaced over the outside of the polyaxial housing so that the posts 30extend through the openings 43 thus allowing the cross-connector to bereceived between the posts and within the slot 28 of the housing. Thecup-shaped cap 40 is arranged to proceed downwardly within the axialbore of the polyaxial housing until the rod engaging surface of thecross-connector comes into contact with the rod 11.

A conventional set screw 48 completes the pedicle screw assembly. Theset screw includes external threads 50, a top 52, and a bottom 58. Thetop 52 has a hexagonal indentation or recess which serves as a wrenchengaging surface 56, i.e., a hexagonal socket. The bottom 58 is flat.When inserted into the axial bore of the polyaxial housing the threadsof the set screw come into contact with the threads on the inner wallsof the posts 30. As the set screw 48 is tightened, the bottom of the setscrew 58 comes into contact with the top of the cross-connector 44forcing the housing upwardly or the rod downwardly or both until the rodis firmly captured between the head of the implanted screw (includingthe washer) and the cap. The outer segments 45 of the cap 40 serve tosupport the posts 30 and prevent the posts from moving outwardly orspreading apart when the set screw is tightened thereby insuring astable assembly.

Since the set screw fits inside of the polyaxial housing, there is nocontact between the set screw and the tissue of the patient. Thisreduces the risk of tissue damage and allows for a more limited area ofsurgical intrusion for the installation of the pedicle screw assembly.Further, the wrench engaging surface is more accessible and requires aless bulky wrench or securing device to accomplish the tighteningprocess. These factors lessen the time required for the operation,minimize tissue damage, and utilizes a smaller securing device to fixthe assembly into position.

Before the final tightening operation, the polyaxial housing is freelyrotatable and angularly displaceable about the head of the implantedscrew. This freedom of movement, to accommodate any bends in the rod, isreferred to herein as three degrees of freedom. As the proper alignmentis achieved, the assembly can be secured in a single desired position bythe final tightening of the set screw.

As a result of this procedure, the bone segments are brought into astable immobilized position. This is best understood by referring toFIG. 1. This figure shows the pedicle assembly and rod in place for theimmobilization of spinal bone segments. The rod is connected to otherpedicle screw assemblies, not shown, and thus keeps the bone segments inan immobilized state.

The components of the screw assembly as well as the rod may be made of ahigh strength material, such as stainless steel, or preferably titanium,which is compatible with the surrounding bone and tissue.

An alternative cap 40′, with a preferred rod engaging surface 47′ and analternative washer design 34′ with a preferred screw head engagingsurface 38′, are illustrated in FIGS. 6-8 and 11. Each of the two sides47 a′ and 47 b′ of the rod engaging surface 47′ of the cap crossconnector 44′ (which straddle an upper center section 47 c′) are formedin an upwardly extending stair step pattern which results in inwardlyprotruding sharp ridges 47 d′. These sharp ridges extend parallel to thelongitudinal axis x—x of the rod and are forced against the rod surfacewhen the set screw is tightened. The sharp ridges inhibit any rotationalmotion or twisting of the rod.

The washer 34′ has a roughened surface formed by stair step pattern ofcircular sharp edges or ridges 38 a′ concentrically arranged around thecentral axis y—y of the housing. The stair step pattern extends upwardlyfrom the bottom of the washer to a central opening 34 b′. The sharpedges are forced against the upper surface of the screw head when theset screw is tightened to fix the angular position of the housingrelative to the screw.

The biting action of the sharp edges 38′ a vis a vis the screw headfirmly locks the housing in place relative to the screw head.

Referring now to FIGS. 9 and 10, there is illustrated an otheralternative design for the rod engaging surface of the cap crossconnector 44″. The cross connector 44″ is formed with a cross sectionalradius R₁, over a central sector S₁, which is substantially the sameradius of the rod. Beginning at the ends 60 of the sector S₁, the radiusis gradually decreased (i.e., over a sector S₂) to a radius R₂ (slightlysmaller than R₁) at the bottom 62 of the recess as is illustrated inFIG. 9. The difference in the radii R₁ and R₂, which need only be a fewthousands of an inch, allows the cross connector 44″ to snap over theouter surface of the rod 11 with an interface fit and effectively andsecurely clamps the cap to the rod when the set screw is tightened. Thisensures that the rod will not rotate, twist or slide longitudinallywithin the housing.

The parameters of the present device may be altered in numerous wayswithout departing from the spirit and scope of the invention as definedin the appended claims. Therefore, it is intended that the drawing beinterpreted as illustrative and not in any way viewed as being alimitation on the invention.

I claim:
 1. A pedicle screw assembly for use in conjunction with a rodfor immobilizing bone segments comprising: a screw having a threadedshaft for insertion into a bone segment and an enlarged head; apolyaxial housing having an upper and lower portion, the head of thescrew being captured in the lower portion of the housing, the housingbeing free to rotate and pivot relative to the head of the screw andhaving a pair of upstanding posts forming the upper portion of thehousing, the posts defining a U-shaped slot therebetween to accommodatethe rod and having interior threads; a set screw arranged to be threadedinto the interior threads of the housing to clamp the assembly to therod; a cup-shaped cap having two opposing openings which fit over theposts and a cross connector extending across the bottom thereof, thecross connector being positioned within the U-shaped slot, between theset screw and the rod, and defining a concave generally saddle-shapedrod engaging surface for engaging the upper surface of the rod, the rodengaging surface being arranged to clamp against the rod when the setscrew is tightened; and a washer positioned within the housing betweenthe rod and the head of the screw, the washer having a concave generallysaddle-shaped upper surface for engaging the lower surface of the rod,the lower surface of cross connector and the upper surface of the washerserving to firmly grasp the rod when the set screw is tightened toprevent the rod from rotating or moving longitudinally within thehousing.
 2. The pedical screw assembly of claim 1 wherein the rodengaging concave surface of the cross connector includes a plurality ofsharp projections which tend to bite into the surface of the rod whenthe set screw is tightened.
 3. The pedical screw assembly of claim 2wherein the rod engaging concave surface of the cross connector isformed with a plurality of sharp ridges extending parallel to thelongitudinal axis of the rod.
 4. The pedical screw assembly of claim 3wherein the rod engaging concave surface of the cross connector hasopposing sides and wherein each side is formed in an upwardly extendingstair step pattern.
 5. The pedical screw assembly of claim 2 wherein therod engaging concave surface of the cross connector forms a recess whichis generally semicylindrical in cross section with the bottom of therecess having a radius which is slightly smaller than the radius of therod, whereby the bottom of the cross connector is forced outwardly tosnap over the outer surface of the rod when the set screw is tightened.6. The pedicle screw assembly of claim 1 wherein the head of the screwhas a generally semispherical upper surface and wherein the washer has agenerally semispherical lower surface for engaging the upper surface ofthe head of the screw.
 7. The pedicle screw assembly of claim 6 whereinthe lower surface of the washer has a roughened surface for grasping thehead of the screw and maintaining the housing in a fixed angularrelationship to the screw once the set screw is tightened.
 8. Thepedicle screw assembly of claim 7 wherein the lower surface of thewasher is formed with a plurality of sharp protruding edges.
 9. Thepedicle screw assembly of claim 8 wherein the sharp protruding edgesform a circular pattern on the lower surface of the washer.
 10. Thepedicle screw assembly of claim 9 wherein the washer defines a bottomand wherein the protruding edges define a stair step pattern ofconcentric circles decreasing in diameter and extending upwardly fromthe bottom of the washer.
 11. A pedicle screw assembly for use inconjunction with a cylindrical rod for immobilizing bone segmentscomprising: a screw having a threaded shaft for insertion into a bonesegment and an enlarged head with a generally convex semispherical topand bottom surface; a polyaxial housing, the housing having a generallyspherical concave inside surface on the lower portion thereof whichcontacts the bottom surface of the head of the screw to allow thehousing to rotate and pivot relative to the head of the screw thehousing including a pair of upstanding posts which form the upperportion of the housing, the posts defining a U-shaped slot therebetweento accommodate the rod and having interior threads; a set screw arrangedto be threaded in to the housing threads to clamp the assembly to therod; a cup-shaped cap defining diametrically opposed openings which fitover the posts so that the cap prevents the housing posts from spreadingapart when the set screw is tightened, the cap having a cross connectorextending across the bottom thereof, the cross connector beingpositioned within the U-shaped slot, between the set screw and the rod,and defining a concave semicylindrical lower surface for gripping theupper surface of the rod; and a washer positioned within the housingbetween the rod and the head of the screw, the washer having a concavegenerally semicylindrical upper surface for gripping the lower surfaceof the rod, and a generally semispherical lower surface for engaging theupper surface of the head of the screw, the lower surface of the crossconnector and the upper surface of the washer serving to firmly graspthe rod therebetween when the set screw is tightened to prevent the rodfrom rotating or moving longitudinally within the housing, the lowersurface of the washer serving to grasp the head of the screw and securethe housing in a fixed position relative to the head of the screw whenthe set screw is tightened.
 12. The pedical screw assembly of claim 11wherein the rod gripping concave surface of the cross connector includesa plurality of sharp projections which tend to bite into the surface ofthe rod when the set screw is tightened.
 13. The pedical screw assemblyof claim 12 wherein the rod gripping concave surface of the crossconnector is formed with a plurality of sharp ridges extending parallelto the longitudinal axis of the rod.
 14. The pedical screw assembly ofclaim 13 wherein the rod gripping concave surface of the cross connectorhas opposing sides and wherein each side is formed in an upwardlyextending stair step pattern.
 15. The pedical screw assembly of claim 12wherein the rod gripping concave surface of the cross connector definesa recess with a bottom, the recess extending upwardly from the bottomand having a generally semicylindrical cross section with a radius R₂ atthe bottom thereof which is slightly smaller than the radius of the rod,whereby the bottom of the cross connector is forced outwardly to snapover the outer surface of the rod when the set screw is tightened. 16.The pedical screw assembly of claim 15 wherein the rod gripping surfaceof the cross connector is formed with a central sector (51) having across-sectional radius R₁ substantially equal to the radius R₁ of therod and end sectors (52) extending between the central sector and thebottom of the recess, the cross-sectional radius of the surfacedecreasing from the ends of the central sector to radius R₂ at thebottom of the recess.
 17. The pedicle screw assembly of claim 11 whereinthe lower surface of the washer has a roughened surface for grasping thehead of the screw and maintaining the housing in a fixed angularrelationship to the screw once the set screw is tightened.
 18. Thepedicle screw assembly of claim 17 wherein the lower surface of thewasher is formed with a plurality of sharp protruding edges.
 19. Thepedicle screw assembly of claim 18 wherein the sharp protruding edgesform a circular pattern on the lower surface of the washer.
 20. Thepedicle screw assembly of claim 19 wherein the protruding edges define astair step pattern of concentric circles decreasing in diameter from thebottom of the washer.